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There is a technique which is used in my amateur radio transmitter for AM on the “low” bands, whereby it is possible to produce a higher power AM signal compared to a conventional AM transmitter with the same power rating and without negative clipping. It is used to provide a more powerful AM signal with modulation while keeping the no audio DC power input to the final low so as not to overheat the finals.
With no audio the DC power input is “a few” (from the manual) watts. With audio, the DC power is increased in proportion to the audio amplitude up to a DC power (the carrier) of around 50 watts with a PEP of 260 watts. The effect is to produce a modulated signal which is nearly equivalent to that of a conventional AM transmitter by boosting the carrier power with the modulation. (This is not double sideband suppressed carrier since the carrier power is increased with the audio.) Since human speech has a high peak to average power ratio, especially with pauses, the average power into the finals is low and they don’t overheat. Keep in mind that in AM the carrier power is constant and the audio is contained in both sidebands but, unlike standard AM, this system increases both the carrier power and the sideband power with audio. The effect is the “few watt” transmitter produces a signal equivalent to that of a 50 watt transmitter.
I recall but can’t find the article where this was described for a part 15 transmitter. Though it appears to be legal under part 15.219 it can be argued that it is not. In the case of my transmitter it is done to prevent overheating the finals rather than because of a legal requirement.
This technique is a function of the transmitter and is not produced by audio processing.
Neil
